Tinned insulated conductors are commonplace in the telecommunications industry. Their use in the field involves the step of stripping their insulation covers with the tin coating remaining to facilitate soldering operations. The tin coating is also helpful in establishing a gas tight seal when a tinned conductor is wrapped about a terminal. Because the tin coating is relatively soft, it also forms a compliant mechanical bond with the terminal in a wire-wrapped connection.
Typically, the metallic portion of the insulated conductor is tinned, then drawn and then heat-treated by annealing prior to insulating in order to cause the metallic conductor to have particular properties. When the metallic conductor has been tinned prior to draw, it is commonly referred to as a pre-tinned conductor. Whenever a pre-tinned conductor is annealed, the relatively high temperature of annealing causes the tin coating to soften which results in an irregular outer surface on the conductor. Because of the irregular interface between the insulation and the conductor, the plastic insulation is caused to have an undesirably high adhesion to the conductor. As one can imagine, this causes problems for installers wishing to strip the insulation to make connections in the field.
The last-mentioned problem becomes aggravated when the final product is one in which the plastic insulation has been irradiation cross-linked after it has been extruded over the wire. For a discussion of irradiation cross-linked insulation and apparatus for making same, see U.S. Pat. No. 3,925,671, which issued in the names of J. R. Austin et al on Dec. 9, 1975 and U.S. Pat. No. 3,623,940 which issued in the names of H. M. Gladstone et al on Nov. 30, 1971.
As a result of crosslinking, the plastic contracts about the conductor and provides additional adhesion over that obtained with non-cross-linked plastic on a conductor having a smooth, unroughened outer surface. When the plastic insulation over pre-tinned conductors is cross-linked, the increase in adhesion which is normally provided by the cross-linking is accentuated.
Not only can the present process result in excessively high insulation-to-conductor adhesion values, but also in ones that are not uniform along the length of the conductor. This can lead to misleading test results and/or require additional field time in discarding those portions of insulated conductors which may have unacceptably high values.
Seemingly, the prior art does not offer a solution to this problem. Nevertheless, a solution is required if the industry is to be able to reap the benefits such as reduced size and improved mechanical properties of an irradiation cross-linked insulation, and yet not impair the strippability of the insulation from its substrate conductor.